Method of coating aluminum or its alloys



Patented Apr. '24, 19T51 METHOD OF COATING ALUMINUM OR ITS ALLOYS Charles C. Cohn, Elkins Park, Pa., assignor to Samuel L. Cohn and Charles C. Cohn, copartners, doing business as Colonial Alloys Company, Philadelphia, Pa.-

No Drawing. Application November 14, 1949, Serial No. 127,157

13 Claiins. (o1, 1486.2)

This invention relates to the production of nonmetallic coatings on aluminum or its alloys by chemical treatment as contrasted with electrochemical treatment commonly known as anodiz- This application is in part a continuation of my application Serial Number 32,530, filed June 11, 1948, now abandoned.

Transparent coatings may be created on aluminum and its alloys by anodic treatment in various known fashions. The disadvantage of such treatment, however, lies in the fact that the aluminum parts to be anodized must be racked in the electrolyticsolution and rather heavy currents must be used to effect the anodizing.

The production of aluminum oxide (hydrated) coatings on aluminum and its alloys may be secured by treatment of the aluminum with various chemicals without the use of electrical current but such treatment is of limited application because it occurs at the expense ofeither visibly etching the aluminum or of creating gray or black coatings instead of clear transparent ones. It is recognized that the production of clear transparent coatings would be highly advantageous.

be etched to a frosted appearance or a surface which has been etched to some desired appearance will be modified to produce an undesirable appearance. Most chemical treatments, furthermore, will produce an opaque coating generally of a gray to black color completely hiding the surface of the metal. chemical treatment of aluminum or its alloys has As a consequence such generally been confined to situations where the appearance of the metal surface is immaterial as, for example, where the treatment is intended merely to supply a paint base or where it is used to provide a dielectric coating. The baths heretofore used'ha've generally been various alkaline chromate bathssometimes containing heavymetals, phosphates, silicates or 'the llike. V In accordance with the present invention'there is provided a chemical treatment which will produce a transparent coating on aluminum or its 'alloys, apparently anoxidic coating, without the 1 disadvantages just indicated. A clean or mechanically, chemically or electrolytically polished surface of aluminum r an alloy thereof may be subjected to the improved treatment with the production of a transparent coating sufiicient' to protect the surface with no substantial impairment of the appearance of the original surface of the metal. This result is attained despite the fact that some of the surface of the metal is necessarily removed by the treatment. In this sense the metal surface is etched; but theetching is not of the usual type resulting in substantial dulling or frosting of the surface, a shiny metallic surface appearance being retained and visible through the coating despite the chemical action which occurs. Furthermore, various pretreatments are known which will produce 0n the surface of aluminum or its alloys desirable uniform fine frosting. Such surfaces subjected to the present treatment also remain substantially unchanged as viewed through the transparent coating which is built up thereon by the treatment. The treatment, furthermore, is readily carried out through the use of inexpensive solutions and, as indicated above, without the necessity for utilizing electrical current so that there is no necessity for racking the parts to be treate for conduction of current thereto.

Under some conditions the coating formed is capable of absorbing dyes to provide colored coatings. Those coatings which, as formed, are not capable of absorbing substantial amounts of dyes, may be treated with acidic solutions as hereinafter described and are then capable of dye absorption to provide deeply colored coatings.

The general object of the invention is accordingly the treatment of aluminum to secure results such as those just indicated. One of the objects of the present invention is the provision of a bath which is capable of effecting the desired treatment. ,These and other objects of the invention will become apparent from the following description of details of carrying out the improved process. p r t The baths which are used are characterized by the fact that they contain lithium, carbonate and chromate ions. The particular other acid. or

chromate ion content and have pH values, measured at 25 C., ranging from 9.5 to about 13.5, for example containing up to 16 grams of free sodium hydroxideper liter, or its equivalent from the standpoint of producing alkalinity. The above ranges represent the limits for operation which may be regarded as fully satisfactory in securing substantially transparent well-adhering coatings. It may be remarked that saturation in lithium content occurs when this content is about 0.25N, more or less, this limit varying to some extent with the amount of other constituents present and being set by the limit of solubility of lithium carbonate in the bath. The more desirable baths have been found to be 0.05 to 0.20N in lithium ion content, 0.35N t 0-45N in carbonate ion content and 0.1'0N to 0.15N in chromate ion content with a pH ranging from 9.5

to 11.0. Still further indicating the baths giving the best results, there may be cited those which are 0.08N to 0.20N in lithium ion content, 0.40N to 0.45N in carbonate ion content, and approximately 0.14N in chromate ion content and having a pH of 10.0 to 11.0.

It may be here noted that the normalities indicated are the usual ones for the ions, i. e., a bath normal in lithium ion would contain one gram atomic weight of lithium per liter, a bath normal in carbonate ion content would contain a gram molecular Weight of the carbonate radical per liter, and a bath normal in chromate ion content (on an oxidation basis) would contain of a gram molecular weight of the chromate radical per liter. It is further to be understood that the contents of ions referred to signify the total of these groups present without implying that the normalities given represent total dissociation of the salts containing these ions.

The baths themselves may be made up in innumerable ways. The lithium may be added, for example, inthe form of sulfate, hydroxide, carbonate or other salts. Carbonate ion may be introduced in the form of carbonates, bicarbonates or sesquicarbonates of potassium, sodium, ammonium or other metals. The chromate ions may be introduced in the form of potassium, sodium, ammonium or other chromates or dichromates. The pH values of baths may be adjusted to the desired ranges by suitable additions of sodium, potassium, ammonium or other hydroxides or alkalies, or by the addition of sulphuric, hydrochloric, nitric, acetic or other acids or acid salts. As has been indicated, the other ions or materials which are introduced have little,

if any, effect on the properties of the baths. It is desirable to add wetting agents to promote uniform action.

As to the pH value of the baths, these are by no means critical but the lower limit of 9.5 represents the point of substantial slowing down of the reaction to a velocity which. renders the bath rather impractical in commercial use, while the value of 13.5 represents an approach to the highest optimum reaction rate which is generally practical at temperatures of'the order of 95 C. since above this value the baths tend to produce, at elevated temperatures, etching in the usual sense of providing a dull or frosted surface on the metal. The upper limit, however, is not critical and depends upon the results desired.

The treatment of aluminum or its alloys with these baths is quite simple. The articles to be treated may be suitably supported, large articles, for example, on racks and small ones in baskets or other containers, and may be then lowered into the baths which are preferably heated to elevated temperatures of the order of 90 C. to boiling. The temperatures are not critical but lower temperatures merely mean that the reaction is slowed down to such extent as to require longer periods of treatment to attain desired results. Temperatures elevated above room temperatures are, however, desirable since at ordinary room temperatures the treatment is quite slow. It may be noted that it is not desirable to try to speed up low temperature treatment by excessive increase of the pH of the bath since while the treatment is speeded up the articles treated are very likely to be etched to an objectionable degree. There is, however, some adjustment possible between pH and temperature: higher pH values may be used if the temperature is kept low, while if the temperature is high, smallerpH values should be used to prevent too great etching. An alkalinity corresponding to 16 grams of sodium hydroxide per liter may be used at temperatures below about C.

The time of treatment is also subject to considerable variation. Five to ten minutes will generally suffice to secure a maximum degree of coating. Greater times are not necessary since they produce little effect, the coating, once it has built up to its normal thickness, acting as a deterrent to further action of the bath. The minimum time is also relatively arbitrary; for example, if it is merely desired to secure a quite thin coating for the purpose of providing a paint base a dip of thirty seconds will .be sufiicient.

The treatment set forth above is quite generally applicable to both substantially pure aluminum and its various commercial alloys. In the cases of some alloys the limits of composition of the solution are somewhat more restricted thanas given by the broadest limits of composition previously set forth; however, the narrower limits are applicable quite generally. Of course, it will be understood that aluminum alloys which contain very'large proportions of other metals may not be coated as satisfactorily as those which more nearly approach pure aluminum, this being true of most treatments which, like the present one, involve some removal of aluminum from the metallic surface, exposing to a greater or less degree the alloying metals which may produce discoloration of the surface.

Coatings produced in accordance with the above are transparent as has been previously indicated and are formed without substantial change of the underlying metallic surface condition whether this condition is produced by chemical, electrolytic or mechanical polishing, or by other means. It may be here noted that while electrolytically or chemically polished surfaces of aluminum or its alloys may be anodized to provide transparent coatings without substantial impairment of the condition of the metal surface, this is not generallytrue in the case of mechanically polished surfaces which are damaged during the anodizing process. The present invention accordingly offers the advantage of making possible the transparent coating of mechanically polished surfaces without substantially damaging them.

The present coating eliminates the necessity for lacquering aluminum surfaces to maintain their condition. The coating is thin but has substantial insulating qualities and is both impervious (when the pH of the bath used is relatively low) and hard so as to provide substantial resistance to both abrasion and corrosion. While the coating has been indicated as particularly colors which are decorative.

-desirable because of the appearance which 'results, it may be noted that if aluminum is coated in accordance with the present process and the coating is then removed by solution in acids the metallic surface which remains forms an excellent base for' plating in accordance with the usual practices. a 1

When the coating is formed under conditions of a pH less than 12 to 13 it is relatively impervious and does not well lend itself to the dyeing of the coated metal to produce deep coloration,

though most of the usual dyes may be absorbed to some extent, giving rise to tinted surfaces of pleasing appearance. If such tinting is "to be accomplished it is desirable to use coat-forming solutions of the types indicated above but in the low range of chromate ion content.

But if the coating is formed with a solution of the type indicated having a higher pH, for example, of about 13 or above, the coating is sufiiciently pervious to absorb, large amounts of dyes to produce deeply colored coatings. The dyes used may be of the usual types and need not be specifically described. Examples are, for

instance, Chrome Fast Orange R which gives a beautiful brass-gold color, and Alizerine Blue "S. A. P. which gives a deep blue.

With long treatments to produce the coatings, thick ones are formed which may provide very deep coloring.

It has further been found that the relatively impervious coatings formed by treatment at low values of pH may be rendered pervious and dyeabsorbent, without adverse efiect on their other properties, by treatment with numerous organic .acids or organic acid saltsin solutions adjusted 5,.

to pH values, at C., in the acidic range from 1 to about 7. For example, treatments with phenol sulphuric acid mixtures are highly effective. The times and temperatures of such treatments are of little significance! dips ranging they are polybasic: phthalic, benzoic, salicylic,

formic, acetic, propionic, succinic, malonic, oxalic, tartaric, citric, glycollic, lactic, gluconic, acrylic, crotonic, tannic, gallic, etc. These are effective .when used in solutions adjusted or buffered to pH values in the acidic range from 1 to aboutfl. Times of treatment ranging upwards of one minute or'less may be used depending upon the depth of color desired and the coating treated. Inorganic acids and inorganic acid salts, however, do not appear to be effective. The concentrations of the acids used to produce the desired effects are not important, the concentrations merely determining the time of treatment. In general, concentrations exceeding about 0.5% are suflicient. is used herein in a general sense to include such actions as adsorption and laking.

The coatings may be colored inorganically by treatment with chromate or permanganate solutions there being thus secured golden to brown The coatings may be after-treated in the various fashions known in the art of anodizing and chemical oxidation, such assealing, pore-plugging, and absorption of oils or other materials, including, of course,

' dyestuffs. I

Precisely what occurs in the treatments in accordance with the invention and particularly The term absorption of dyes.

the function of the lithium is not known. Analysis of the coatings reveals the presence of small amounts of lithium, for example up to about 0.1%, but it has so far been impossible to ascertain, due to the small amounts of lithium and the thinness of the coating, whether the lithium is merely absorbed from the bath into a coatingprimarily of hydrated aluminum oxide or whether the lithium is present in some form combined with aluminum as, for example, in the form of an aluminate. Extensive investigation has shown, however, that to secure the desired results lithium must be present in the bath and cannot be equivalently replaced by other metals, even the alkalies which are generally considered to be closely the equivalents of lithium.

What is claimed is:

1. The method of producing an aluminum oxide coating on aluminum or its alloys containing aluminum as their major constituent including treating a surface of such metal with an aqueous solution containing lithium, carbonate and chromate ions as its essential constituents, said solution being 0.01Nt0 saturated in lithium ion content, 0.06N to 0.80N in carbonate ion content, and 0.0'7N to 0.60N in chromate ion content, and which has a pH of not less than about 9.5.

2. The method of producing an aluminum oxide coating on aluminum or its alloys containing aluminum as their major constituent including treating a surface of such metal with a hot aque ous solution containing lithium, carbonate and chromate ions as its essential constituents, said solution being 0.01N to saturated in lithium ion content, 0.06N to 0.80N in carbonate ion content, and 0.0'7N to 0.60N in chromate ion content, and which has a pH of not less than about 9.5.

3. The method of producing an aluminum oxide coating on aluminum or its alloys containing aluminum as their major constituent including treating a surface of such metal at a temperature within the range of 90 Cfto 100 C. with an aqueous solution containing lithium, carbonate and chromate ions as its essential constituents, said solution being 0.01N to saturated in lithium ion content,'0.06N to 0.80N in carbonate ion content, and 0.07N to 0.60N in chromate ion content, and which has a pH of not less than about 9.5.

4. The method of producing an aluminum ox- :ide coating on aluminum or its alloys containing aluminum as. their, major constituent including treating a surface of such metal with an aqueous solution containing lithium, carbonate and chromate ions as its essential constituents, said solution being 0.05N to 0.20N in lithium ion content, 0.35N to 0.45N in carbonate ion content, and 0.10N to 0.15N in chromate ion content, and which has a pH of not less than about 9.5.

5. The method of producing an aluminum oxide coating on aluminum or its alloys containing aluminum as their major constituent including treating a surface of such metal with a hot aqueoussolution containing lithium, carbonate and chromate ions as its essential constituents, said aluminum as their major constituent including treating a surface of such metal at a temperature within the range of C. to C. with an aqueous solution containing lithium, carbonate and chromate ions as its essential constituents, said solution being 0.05N to 0.20N in lithium ion content, 0.35N to 0.45N in carbonate ion content, and 0.10N to 0.15N in chromate ion content, and which has a pH of not less than about 9.5.

7. The method of producing an aluminum oxide coating on aluminum or its alloys containing aluminum as their major constituent including treating a surface of such metal with an aqueous solution containing lithium, carbonate and chromate ions as its essential constituents, said solution being 0.08N to 0.20N in lithium ion content, 0.40N to 0.45N in carbonate ion content, and approximately 0,14N in chromate ion content, and which has a pH of not less than about 10.0.

8. The method of producing an aluminum oxide coating on aluminum or its alloys containing aluminum as their major constituent including treating a surface of such metal with a hot aqueous solution containing lithium, carbonate and chromate ions as its essential constituents, said solution being 0.08N to 0.20N in lithium ion content, 0.40N to 0.45N in carbonate ion content, and approximately 0.14N in chromate ion content, and which has a pH of not less than about 9. The method of producing an aluminum oxide coating on aluminum or its alloys containing aluminum as their major constituent includin treating a surface of such metal at a temperature within the range of 90 C. to 100 C. with an aqueous solution containing lithium, carbonate and chromate ions as its essential constituents, said solution being 0.08N to 0.20N in lithium ion content, OAON to 0.45N in carbonate ion content, and approximately 0.14N in chromate ion content, and which has a pH of not less than about 10. An aqueous bath for the surface treatment of aluminum or its alloys containing aluminum as their major constituent and adapted to produce a clear aluminum oxide coating thereon by chemical action which contains lithium, carbonate and chromate ions as its essential constituents, and is 0.01N to saturated in lithium ion content, 0.06N to 0.80N in carbonate ion content, and 0.07N to 0.60N in chromate ion content, and which has a pH of not less than about 9.5.

11. An aqueous bath for the surface treatment of aluminum or its alloys containing aluminum as their major constituent and adapted to produce a clear aluminum oxide coating thereon by chemical action which contains lithium, carbonate and chromate ions as'its essential constituents, and is 0.05N to 0.20N in lithium ion content, 0.35N to 0.45N in carbonate ion content, and 0.10N to 0.15N in chromate ion content, and which has a pH of not less than about 9.5.

12. An aqueous bath for the surface treatment of aluminum or its alloys containing aluminum as their major constituent and adapted to produce a clear aluminum oxide coating thereon by chemical action which contains lithium, carbonate and chromate ions as its essential constituents, and is 0.08N to 0.20N in lithium ion content, 0.40N to 0.45N in carbonate ion content, and approximately 0.14N in chromate ion content, and which has a pH of not less than about 10.0.

13. The method of treating aluminum or its alloys containing aluminum as their major constituent including producing an aluminum oxide coating on such metal by treating a surface of such metal with an aqueous solution containing lithium, carbonate and chromate ions as its essential constituents, said solution being 0.01N to saturated in lithium ion content, 0.06N to 0.80N in carbonate ion content, and 0.07N to 0.60N in chromate ion content, and which has a pH of not less than about 9.5, and then treating such coated metal with an aqueous solution of an organic acid having a pH in the acidic range from 1 to about 7 to increase its dye absorption properties.

CHARLES C. COHN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,001,427, Leahy May 14, 1935 2,153,060 Guthrie Apr. 4, 1939 FOREIGN PATENTS Number Country 1 Date 535,199 Great Britain Apr. 1, 1941 OTHER REFERENCES Handbook of Chemistry and Physics, 27th edition, page 1350, publ. 1943 by Chemical Rubber Publishing Co., Cleveland, Ohio.

Jones, Inorganic Chemistry, pages 244, and 301 to 303, publ. 1947 by the Blakiston Co., Philadelphia, Pa. 

